The present invention is directed to the use of ultraviolet resonance Raman (UVRR) spectroscopy and membrane filtration techniques for the development of an on-line monitoring and process control system that will improve the reliability and performance of biological nutrient removal (BNR) wastewater treatment plants (WWTPs). This monitoring system enables real time in situ measurement of nitrate and nitrite in BNR activated sludge reactors without the need for reagent additions or complex calibration procedures. Real time on-line monitoring of these parameters can provide input to a process control system used to optimize performance of treatment systems designed for low effluent concentrations of both nitrogen and phosphorus. An additional benefit will be the reduction of energy consumption for process aeration.
This system can be readily applied to the monitoring and control of simultaneous nitrification and denitrification (SNdN), and would be particularly applicable to the control of the relatively new membrane bioreactor (MBR) treatment processes which are rapidly gaining interest for BNR and water reclamation treatment facilities.
The generally accepted energy yielding two-step oxidation of ammonia to nitrate is as follows. (Randall et al., 1992):
Nitrosomonas2NH4++3O2→2NO2−+4H++2H2O  Equation 1Nitrobacter2NO2−+O2→2NO3−  Equation 2The total reaction isNH4++2O2→NO3−+2H++H2O  Equation 3
The total reaction shows that 4.57 g O2 is required per g-NH4+—N oxidized, but when the nitrogen used for cell syntheses is included the oxygen requirement is 4.3 g O2/g-NH4+—N oxidized to nitrate (Randall et al. 1992). Temperature, dissolved oxygen (DO) concentration, and ammonia nitrogen concentration all affect nitrification rates. Nitrification kinetics is normally based on ammonia concentrations since nitrite is oxidized rapidly under fully aerobic conditions. The actual growth rate of nitrifiers (μN) can be expressed as a function of ammonia and DO by a Monod kinetic equation:μN=μNmax×[(NH4+—N)/(KN+NH4+—N)]×[(DO)/(Ko+DO)]  Equation 4
At very low DO levels the term [(DO)/(Ko+DO)] approaches zero and nitrification will no longer occur. Under long periods of very low DO conditions the nitrifiers will be lost from the system.
Nitrate reduction in wastewater systems occurs through assimilation and denitrification. In assimilatory nitrate reduction, nitrate is reduced to ammonia and assimilated for cell synthesis. In denitrification bacteria use nitrate as an electron acceptor in the absence of oxygen to oxidize an organic or inorganic electron donor. Nitrate is reduced to nitrite, to nitric oxide, to nitrous oxide, and to nitrogen in a four-step process (Payne et al, 1981):NO3−→NO2−→NO→N2O→N2  Equation 5
The rate of denitrification (RDN) is dependent on temperature and DO concentration, where K is the temperature correction coefficient which is commonly assumed to be 1.09.RDN(T)=RDN(20)×K(T-20)×(1−DO)  Equation 6
Equation 6 shows that the rate of denitrification decreases linearly from 0 to 1 mg/l of DO. At DO levels of 1 mg/l and above the rate of denitrification becomes negligible. From equations 4 and 6, it is apparent that in order to have conditions which support SNdN it is necessary to maintain the DO concentration within a very narrow range.
Aeration control methods based on a DO measurement feedback loop have proven to be inadequate to achieve reliable and consistent SNdN operating conditions. The problems with DO measurement alone are twofold. The first problem is that DO instrumentation has historically been unable to provide the necessary accuracy at the very low DO levels required to achieve SNdN (typically in the range of 0 to 0.3 mg/l). A second and more fundamental problem is that the DO measurement alone is not sufficient to define the nitrification and denitrification metabolic conditions within the reactor. Nitrification and denitrification rates are a function of DO, temperature, pH, solids retention time (SRT), microbial population dynamics, and other factors. For example, one system under a given set of conditions may lose nitrification at a DO level of 0.3 mg/L while another system may continue to nitrify when the measured DO level in the bulk solution is very near 0 mg/L.
Other instrumentation and control methods have been applied to attempt control of SNdN process reactors including nicotinamide adenine dinucleotide (NADH) sensors, oxidation reduction potential (ORP) electrodes, and on-line nitrate analyzers. Each of these methods has significant limitations in the application to SNdN control due to cost and/or performance issues.
The performance and cost effectiveness of the measurement and control method proposed here will greatly exceed those of the currently available methods for SNdN control. Our proposed method will use UVRR for direct simultaneous measurement of both nitrate and nitrite. The control algorithm will be based on the calculated ratio of nitrite to nitrate.
Recent research using laboratory bench scale reactors (Kuai and Verstraete, 1998) has demonstrated that the nitrite oxidizers responsible for conversion of nitrite to nitrate are strongly inhibited by low DO concentrations. In this work NO2−/(NO2−+NO3−) ratios were found to be in the range of 0.9 to 1.0 under oxygen limiting conditions. Under fully aerobic conditions nitrite concentrations are typically very low and the corresponding ratio of NO2−/(NO2−+NO3−) ratio will also be very low (<0.1).
Other researchers have also observed that at very low DO concentrations a “Nitrite Shunt” may be occurring (O'Neill and Huren, 1995), in which nitrite is produced from nitrification without nitrate formation due to inhibition of the nitrite-oxidizing bacteria by the low DO concentration. The removal of ammonia under these conditions is assumed to occur in a two step process as follows:2NH4++3O2→2NO2−+4H++2H2O  Equation 72NH4++2NO2−→2N2+4H2O  Equation 8With the overall reaction being:4NH4++3O2→2N2+6H2O+4H+  Equation 9
Based on the above stoichiometry, the removal of ammonium nitrogen via the nitrite shunt pathway results in a 63% energy savings versus conventional nitrification processes (Kuai and Verstraete, 1998) and nearly 40% versus conventional nitrification-denitrification processes.
Regardless of whether SNdN is a result of the formation of anoxic regions within an otherwise aerobic environment due to incomplete mixing, a result of diffusional limitations on the oxygen transfer into the floc (causing an anoxic region within the floc itself), a distinct biological phenomenon (such as the “nitrite shunt” pathway), or a combination of these factors, the measurement and control system that we propose is ideally suited to maintaining conditions in a reactor to maximize the potential for reliably and efficiently achieving SNdN. The algorithm for aeration system control to achieve SNdN using our system would utilize high and low setpoints for the ratio of nitrite to nitrate with the objective of maintaining DO levels as low as possible without losing nitrification. Where possible, designers and operators of BNR treatment facilities may elect to establish setpoint values which maximize the potential for promoting the nitrite shunt pathway to achieve additional energy savings. The widespread implementation of a monitoring and control system with these capabilities can be expected to lead to significant progress in the understanding of SNdN metabolic pathways and practical application methods.
Raman spectroscopy is the measurement of the wavelength and intensity of inelastically scattered light from molecules. The Raman scattered light occurs at wavelengths that are shifted from the incident light by the energies of molecular vibrations. Light photons which are shifted to a lower energy (longer wavelength) are said to be Stokes shifted while a shift to a higher energy (shorter wavelength) is termed an anti-Stokes shift. Elastic photon collisions result in Rayleigh scattering, where the energy of the photon is the same before and after the collision. In Rayleigh scattering, only the photon's direction of travel has changed.
Raman spectroscopy was first discovered in 1928 by C. V. Raman (Raman and Krishnan, 1928), but in recent years has been revolutionized by several new technological developments, resulting in enormous increases in Raman signal detection capabilities, lower instrumentation costs, and relatively compact equipment that permits mobile, on-site measurements. Raman spectroscopy, compared to competing techniques for chemical identification, has minimal requirements for sample preparation and requires no reagent additions. Raman spectroscopy can readily be used in aqueous systems since the water molecule has very weak Raman activity. These factors have led to a growing interest in Raman spectroscopy in industry for chemical identification and for on-line monitoring and control. This rapidly emerging technology offers significant opportunities for development of applications in the monitoring and control of wastewater treatment processes.
The intensities of Raman shift spectral bands are only a very small percentage of the excitation light source which makes their detection and measurement experimentally difficult. Nitrate and nitrite have Raman shift lines due the symmetrical N—O stretch vibrational mode at 1044 cm−1 and 1325 cm−1 respectively (Laane and Ohlsen, 1980) which represent the reduction of energy of the observed line from the incident light wave number where both are given in cm−1.
The intensity of a Raman shift line is proportional to the fourth power of the excitation source frequency. Therefore, using a light source in the red region produces relatively low intensity lines for the symmetrical N—O vibration Raman shifts. A shorter wavelength (higher frequency) excitation source in the green or blue region can be obtained by using, for example, a Nd:YAG laser at 532 nm. The higher excitation frequency will increase the intensity of the Raman shift line, however the problem with doing this is that many organic compounds including many of those found in wastewater, exhibit strong fluorescence of blue and green wavelengths making it impossible to get good signal to noise Raman spectra.
Going to even shorter wavelengths into the UV and deep UV region will further increase the intensity of the Raman shifts due to the effect of the higher frequency excitation. It also avoids the potential interference from fluorescence since condensed phase species typically show no fluorescence below 260 nm (Asher, Munro, and Chu 1997, Asher and Johnson 1985). Even more significantly, since nitrate and nitrite are both strong absorbers of UV light at around 200 to 220 nm, it becomes possible to take advantage of the resonance Raman effect. Resonance Raman scattering requires excitation within an electronic absorption band and results in a large increase of scattering. The resonance Raman scattering can be up to 108 times that of “normal” Raman scattering. (Asher 1993, and Dudik, et al., 1985).
Wastewater treatment plant managers and operators are now facing increasingly stringent regulations, more demanding reporting requirements, pressure to hold down costs, and requirements to improve the treatment performance, particularly in the area of nutrient removal. Improved on-line monitoring and automation of process controls are critical in meeting these challenges. According to the Water Environment Research Foundation (WERF), poor sensor performance and high maintenance costs have been considered the major stumbling blocks to improved automation. (Water Environment Research Foundation, 1998)
Nutrient removal is a significant concern for municipal discharges in many areas of the United States and around the world. The most common, widely accepted and economical approach to accomplish nitrogen removal involves biological nitrification and denitrification. Nitrogen enters municipal wastewater treatment plants in the form of organic nitrogen and ammonia, and during nitrification, the nitrogen is oxidized by autotrophic bacteria to nitrite and nitrate nitrogen. During biological denitrification, biological reactors are operated without oxygen addition, so that the bacteria use nitrate as an electron acceptor for their respiration. Various designs are used, and one of the most common approaches employs an anoxic tank (anoxic meaning bacteria respiration with nitrate in the absence of oxygen) ahead of aeration where recycled nitrate is contacted with influent wastewater to promote nitrate reduction to nitrogen gas.
Where phosphorus removal is also required, the historical approach and one that is still used at most municipal WWTPs is to add aluminum or iron salts to the wastewater treatment process. This method has many disadvantages which include: 1) chemical cost; 2) increased sludge production and difficulty in sludge dewatering; 3) consumption of additional energy for chemical production and transportation to the site; and 4) production of a greater quantity of waste for ultimate disposal. An alternative approach that has received increased acceptance in the last fifteen years is the use of combined biological nitrogen and phosphorus removal processes.
Biological phosphorus removal designs are commonly coupled with nitrogen removal designs by adding an anaerobic contact zone before the first anoxic zone, as shown in FIG. 1, to promote the growth of phosphate accumulating bacteria. The flow scheme depicted here is for the Bardenpho BNR process.
The major limitation of the above biological nutrient removal systems, and other similar BNR designs, is the inability to consistently achieve effluent phosphorus concentrations as low as is necessary to meet most discharge permit limitations (usually less than or equal to 1.0 mg/L as P). In some cases chemicals are added for polishing to lower the phosphorus concentration from that achieved by the biological process, but where this would be necessary it often discourages designers from using biological phosphorous removal in the first place. (Daigger, 1991)
The treatment efficiency limitations of current biological phosphorus removal processes are due to two causes. The first cause of lower phosphorus removal is a low concentration of influent soluble biochemical oxygen demand (BOD). Sufficient BOD is needed to produce the necessary acetate to form polyhydroxy-butyrate (PHB) which is needed in the aerobic zone for phosphorus uptake. This problem is overcome by providing supplemental acetate or sugar to the anaerobic zone. In some cases primary clarifier solids are fermented to provide this necessary acetate.
The second cause of lower phosphorus removal is the occurrence of extended anaerobic contact time where acetate is not available. In this case phosphorus is released without concurrent acetate uptake and PHB formation, making insufficient electron donor present during aeration to provide energy for uptake and formation of polyphosphates from all of the released phosphorus.
Stephens and Stensel (1998) reported that only 40 to 60% of the phosphorus released during anaerobic contacting following nitrate depletion, and without acetate, was taken up during aeration in a sequence batch reactor process during laboratory experiments. Barnard (1984) had hypothesized this phenomena earlier and termed it “secondary phosphorus release”, but only recently has experimental data been presented to indicate its existence and negative impact on biological phosphorus removal.
The potential for “secondary phosphorus release” is great in BNR systems since full-scale facilities are not normally operated at their future maximum expected design loadings. This means that the anoxic zones are larger than needed for much of the operation which will encourage periods of anaerobic conditions (lack of nitrate and oxygen) thus promoting secondary phosphorus release. The problem may also be related to diurnal or seasonal load changes such that periods of excess anoxic zone capacity exist at different times resulting in periodic secondary phosphorus release. Since the anoxic tank capacity must be available to meet higher loads, a monitoring and control system is needed that can respond to changing plant loads and prevent secondary phosphorus release. The key technological development needed to implement a control strategy based on these principles is a cost effective and reliable sensor system capable of monitoring nitrate and nitrite down to 1 mg/L or less.
There is also a great deal of interest currently in developing process designs and control strategies capable of achieving simultaneous nitrification and denitrification. By maintaining very long mean cell residence times, sometimes through the use of immobilization techniques or membrane separation processes, and close control over dissolved oxygen levels, it is possible to achieve ammonia oxidation to nitrite with the nitrite being reduced to nitrogen gas in the same reactor. In these systems the dissolved oxygen (DO) levels must be maintained at very close to zero making it impossible to use a DO feedback loop for control of the aeration system. The ability to obtain real time in situ measurements of nitrate and nitrite in the reactors would significantly enhance the potential application of this type of process. The ability to achieve nitrification and denitrification in one reactor could be a particularly attractive option for upgrading existing treatment facilities to meet more stringent total nitrogen effluent limits.
With increased emphasis on the reuse of treated effluent for land application or groundwater recharge, it becomes necessary to optimize total nitrogen removal in both new and existing facilities. The application of the online nitrate/nitrite monitoring and control system to a multistage nitrification/denitrification facility would be similar to that described above for control of secondary phosphorous release, except that it would be used to adjust the anoxic and aerobic zone operating conditions under varying loading conditions to maximize total nitrogen removal.
Nitrate analyzers which are based on dual beam UV absorption spectroscopy are offered by several manufacturers. These analyzers measure absorption of the samples at two different wavelengths, typically 210 nm and 250 nm. The absorption at 210 nm can be related to the concentration of NOx (NO2− plus NO3−) and the absorption at 250 nm is used to correct for the presence of organic compounds which also exhibit some absorption at 210 nm. This method has two significant drawbacks. First, it cannot differentiate nitrate from nitrite and is, therefore, not suitable for determination of the nitrite to nitrate ratio as is necessary to maintain low DO operating conditions. The second major drawback is the potential for interference from organic compounds present in the wastewater requiring that the instrument be calibrated based on the individual wastewater characteristics at each treatment plant installation.
Other methods exist for nitrate and nitrite analysis including automated analyzers using reagent based spectrophotometric methods and adaptations of these reagent based methods using flow injection analysis (FIA). These methods can be relatively expensive, require regular replacement of reagents, frequent calibration and periodic cleaning and/or replacement of pumps and tubing. For application to the problem of on-line monitoring and control, these methods also have limitations with respect to the length of time required to collect a sample and make a measurement that could be used as an input to a real time process control system.
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